The optical coupling of light emitted, absorbed or altered by optoelectronic (OE) devices, such as photodetectors, light emitting diodes (LED's), lasers, and vertical cavity surface emitting lasers (VCSEL), with optical waveguides, such as optical fibers and planar waveguides, is well known in conventional photonics. One technique that is known involves cutting an optical fiber at a 45 degree bevel so that light is exchanged between the fiber and an OE device at a side of the fiber. The bevel surface may be coated to be more reflective, or it may be left exposed so as to reflect light by total internal reflection (TIR).
The cost of manufacturing such waveguide-to-device couplings is determined by the ease of preparing the both the waveguide (e.g. fiber) and the OE device for the step of coupling, and then performing the coupling itself in a manner that ensures an efficient transfer of optical signal without introduction of noise.
An optoelectronic chip, containing a device such as a vertical cavity surface emitting laser (VCSEL), is typically mounted in an electronic package where the direction of the light from the VCSEL is perpendicular (normal) to the surface of both the chip itself and the surface on which the electronic package has been placed. Electronic packages are typically placed on large 2-D flat printed circuit boards (PCBs), and these PCBs are typically stacked within a chassis with very narrow gaps between the PCBs. This type structure requires that all the connections to and from the PCB enters and leaves from the PCB's edge, called the card-edge. Since the light from the VCSEL is emitted perpendicular to the PCB, a method is required to direct the light off the edge of the PCB, and hence parallel to the flat surface of the PCB. The typical method used to achieve card-edge connections with light is to use a flexible-PCB bent at 90-degrees where one face of the flexible-PCB connects to the main PCB and the other face has the optoelectronic chip where the light from the VCSEL is directed parallel to the surface of the main PCB. The light is then butt-coupled into an optical fiber.
The bevel coupling method allows the optoelectronic chip to be placed in the conventional packages where the light is directed perpendicular to the PCB. The optical fiber is then beveled at 45-degrees and placed over the light beam such that the light is reflected at 90-degrees and propagates parallel to the PCB within the optical fiber. This method allows more conventional packaging and reduces the alignment tolerance because the length of the optical fiber is essentially laid over the flat surface of the PCB.
Several patents use 45-degree beveled optical fiber as the core of their assemblies as well. U.S. Pat. No. 4,092,061 granted May 30, 1978, U.S. Pat. No. 6,250,820 granted Jan. 26, 2001, U.S. Pat. No. 6,315,464 granted Nov. 13, 2001 and U.S. Pat. No. 6,389,202 granted May 14, 2002 all describe assemblies that have beveled optical fiber tips located over (or under) optoelectronic devices. The alignment procedures for these types of assemblies are complicated. These methods typically involve micro-solder ball re-flow, flip-chip alignment and/or precisely machined parts, which require significant resources and materials.
The concept of creating a completely integrated assembly that holds both the optoelectronic devices and the waveguides has also been proposed in U.S. Pat. No. 4,611,886 granted Sep. 16, 1986. It describes a method of using a molded housing that carries a glass-plate with a beveled end, which is aligned using etched grooves in the molded housing that match the chip carrier. This technique may be adequate for large area optoelectronics, but would not be a suitable alignment methodology for small devices such as VCSELs.
Another assembly proposed in U.S. Pat. No. 4,756,590 granted Jul. 12, 1988 describes a method of using a 45-degree bevel-polished silicon v-groove sandwich of optical fibers that has optoelectronic devices glued over the bevel in line with the optical fibers. In the principal embodiment, the optical fibers are held in a block that is polished and beveled. The block is typically made from two silicon v-groove chips that sandwich the optical fibers between them. By polishing the end of the sandwich at 45-degrees and then applying a metallic mirror, the light is forced to reflect at 90-degrees and travel perpendicularly from the optical fibers through one of the silicon v-groove chips. It is, however, unclear how any measureable amount of light (for example: 1-milliwatt of 850-nm wavelength light—typical of a vcsel) can pass through a silicon v-groove chip since silicon is opaque. U.S. Pat. No. 4,756,590 also teaches the removal of part of one silicon v-groove chip by polishing until the longitudinal sides near the tips of the optical fibers are exposed. This is to allow closer access to the core of the optical fiber at the tip.
Finally, U.S. Pat. No. 4,756,590 describes that the optoelectronic devices must be glued against the silicon v-groove ferrule above the optical fiber in a face-down orientation. This completely rules-out a VCSEL chip since the vertical cavity laser would be damaged if bonded up-side-down, not to mention that the wirebond connections are made on the same surface as the vertical cavity laser and it would be physically impossible to wirebond to the VCSEL chip in such an orientation.